Monoalkenyl arene/conjugated diene block copolymers are widely used in pressure sensitive adhesives (PSA). PSA based on these polymers have high strength and elasticity at ambient temperatures, making them well suited for use in many general purpose applications, and in packaging and cloth tapes. The high strength and elasticity of these PSAs is due to the well known microphase separated network structure in which the monoalkenylarene endblocks, phase separate to form domains serving to physically crosslink the rubbery midblock phase. However, at temperatures approaching the glass transition temperature of the endblocks or in the presence of an appropriate solvent, the domains soften, releasing the physical crosslinks and the PSA loses its strength and elasticity. Therefore, PSA based on a block copolymer are unsuitable for use in high temperature or solvent resistant tapes, such as automobile masking tapes. The only method of maintaining high cohesive strength in a PSA based on a block copolymer at high temperature or in the presence of solvent is to chemically crosslink the polymer in order that the polymer no longer depends on the physical crosslinks for its strength.
Heinz et al U.S. Pat. No. 4,320,188 discloses a photo printing plate which can be processed at room temperature and which contains endblocks with a glass transition temperature below room temperature. In the composition according to the Heinz disclosure, both phases are elastomeric and neither is a thermoplastic. It is desired in the present application to have a true thermoplastic rubber which contains an amount of unsaturation in the endblock sufficient to facilitate crosslinking but not sufficient to adversely affect the thermoplastic properties of the polymer. The compositions of the present invention are hot melt adhesives which must be processed at 150.degree. C. and a high glass transition temperature is needed so that phase separation can occur at that temperature.
U.S. Pat. No. 4,133,731 claims that the block polymer in a PSA can be crosslinked chemically by including a multifunctional acrylate or methacrylate crosslinking agent in the PSA formulation and exposing the adhesive to high energy radiation such as electron beam (EB) or ultraviolet (UV) radiation. This approach was successful in chemical crosslinking the polymer in the PSA. However, the aggressive tack of the PSA became poorer when the adhesive was crosslinked. In this case, it is apparent that crosslinking occurred in the rubber midblock phase of the polymer. This would be expected to increase the modulus of the adhesive and, in a PSA, to cause reduction in the aggressive tack of the PSA.
We have discovered that a better approach to chemically crosslink the block copolymer in the PSA is to crosslink the polymer through the endblock phase. That is, if the block polymer can be crosslinked through the endblocks, then the polymer can be converted from a thermoplastic to a thermoset with very little change in modulus of the PSA since modulus is determined by the endblock polymer content of the block polymer and by the chain entanglement density of the rubber used in the midblock of the block polymer. As will be shown, there are two requirements which must be met in order to assure that at least part of the crosslinking reactions will occur in the endblock phase. First, the endblocks must contain carbon to carbon double bonds in order that the free radical crosslinking reaction can occur. Second, a crosslinking agent must be included in the formulation which thermodynamically is more compatible with the endblock phase than with the midblock phase and therefore will preferentially concentrate in the endblock phase.